Time-Travel : Fact or fiction?

Time travel fiction tends to spin our concepts of cause and effect. In fiction and physics, these spins are paradoxes – self-contradictory time loops. Following are four of the most popular time-travel paradoxes. Each is a series of events that loops back on itself to form a dilemma in which an effect blocks its own cause or vice versa.

Grandfather Paradox:

Suppose you construct a time machine and go back to the 1970s and prevent your grandfather from meeting your grandmother. If you succeed, one of your parents would never have been born – and hence you would never have been born. But if you were never born, then you could not have gone back in time to prevent anything from happening. This is the heart of the Grandfather Paradox: a cause (your time travel) is "removed" by its own effect (your non-existence). Simply put, it's like deleting the first line of a story that tells the rest – without it, the story can't even start.

Analogy: It is like you push the first domino in a row of toppling dominoes. Unless the first one falls, none of the rest will fall either – but setting the last one over (the effect) does seem to be contingent on having set the first one over (the cause) to fall. The Grandfather Paradox illustrates that altering the past (murdering the "first domino" in your family history) is contrary to your existence.

Bootstrap Paradox:

The Bootstrap Paradox (or information loop) is the opposite: an object, individual or item of information moves back in time and becomes the very origin of itself. That is, something has a time loop without an initial creator. For instance, suppose you discover the musical score of a famous composer in the future and return it to the composer. The composer subsequently releases it as his own. Who really composed the music? The score "has no beginning" – it appears to have materialized out of thin air. In physics terms, the cause (the travel back) produces the effect (the published score) and vice versa.

Analogy: Imagine handing Shakespeare a copy of his own play that you had borrowed from the future. Shakespeare publishes it, and then you learn it from his book – never having written it yourself. The play is in a loop: you learned it from Shakespeare, but Shakespeare only had it because you gave it to him. This is precisely the Bootstrap Paradox (named after "pulling yourself up by your bootstraps"): the source of the item or information disappears, and it just loops in time.

Polchinski's Paradox:

Polchinski’s Paradox is a thought experiment involving a billiard ball and a wormhole (a hypothetical tunnel through spacetime). Imagine a billiard ball rolling toward a wormhole at just the right angle so that the wormhole sends it back in time. The ball emerges a few moments earlier and strikes its younger self, knocking it off course so it never enters the wormhole. If the younger ball never passes through the wormhole, then the older ball never could have returned to strike it – an obvious contradiction. In short, the future self of the ball prevents its past self from ever being in that future. This paradox demonstrates a self-defeating cycle: the motion of the ball through time halts its own cause.

Analogy: It's like tossing a ball to a mirror so it reflects back and prevents you from throwing it to begin with. If the mirror-ball hadn't reflected back to keep you from throwing it, you would have tossed it – but as soon as it keeps you from throwing it, you never toss it, so the ball never bounces back. Polchinski’s Paradox captures a similar impossibility: a future object interferes with its own past in a way that makes its future trip impossible.

Causal Loop (Predestination Paradox)

A causal loop (sometimes called a predestination paradox) is when an event is both the cause and effect of itself, creating a closed loop in time. For instance, suppose a time traveler obtains tomorrow’s winning lottery numbers and uses them to win. Later in the day, the traveler returns to the past and informs their younger self of the winning numbers. Who initially created the numbers? Each event leads to the other in a cycle. In a causal loop, events or information have no cause – the future event is the past cause, and the cycle perpetuates itself.

Analogy: Consider a science fiction scenario in which someone reads a diary full of information about their life, then goes back in time and dictates that same diary to their younger self, who matures and writes the same diary. The diary exists due to itself. Causal loops are the same: a resolution or action feeds back and generates itself. The loop is reliable (it can occur) but it violates our common perception of "before and after."

Conclusion: 

These paradoxes present the bizarre reasonableness of time travel. Each one – Grandfather, Bootstrap, Polchinski, and Causal Loop – contains events that loop around themselves in unphysical ways. Physicists comment that if time travel into the past were ever feasible, nature would have to exclude these paradoxes (for instance, by prohibiting alterations of the past). For the time being, time travel exists in the world of fantasy, where it is an intriguing enigma: a matter of what if? in the web of cause and effect.

References

Deutsch, D. (1991) ‘Quantum mechanics near closed timelike lines’, Physical Review D, 44(10), pp. 3197–3217.

Earman, J. (1995) Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes. Oxford: Oxford University Press.

Lewis, D. (1976) ‘The paradoxes of time travel’, American Philosophical Quarterly, 13(2), pp. 145–152.

Nahin, P. J. (1999) Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction. New York: Springer.

Visser, M. (2003) ‘The quantum physics of chronology protection’, in Novello, M. and Bergliaffa, S.P. (eds.) Proceedings of the 12th Brazilian School of Cosmology and Gravitation. New York: AIP, pp. 163–176.